Re: Original design approach to Oracle REDO Logs
Date: Fri, 18 Jun 2021 18:27:33 +0000
Message-ID: <BN6PR01MB25478208D38FA4F71EA60380CE0D9_at_BN6PR01MB2547.prod.exchangelabs.com>
NO_DATA_NEEDED? I had to look that one up. For anyone else who does not remember ever encountering this, the following might be of interest if you have access to Oracle support.
Pipelined Function Terminates Unexpectedly without Reporting an Error when NO_DATA_NEEDED or OTHERS Exception are not used. (Doc ID 1466789.1) Under what Circumstances will a Pipleined PL/SQL Function Cause ORA-06548? (Doc ID 1059985.1)
Mark Powell
Database Administration
(313) 592-5148
From: oracle-l-bounce_at_freelists.org <oracle-l-bounce_at_freelists.org> on behalf of Michael D O'Shea/Woodward Informatics Ltd <woodwardinformatics_at_strychnine.co.uk> Sent: Friday, June 18, 2021 1:44 PM
To: frankagordon_at_gmail.com <frankagordon_at_gmail.com> Cc: jlewisoracle_at_gmail.com <jlewisoracle_at_gmail.com>; ORACLE-L <oracle-l_at_freelists.org> Subject: AW: Original design approach to Oracle REDO Logs
Would that break some existing applications?
I think so.
This prospective behavioural change, or breaking change if you prefer, could be circumvented by a developer, just as it was possible for the developer to change the codebase to swallow the (silly) NO_DATA_NEEDED error and work around the disappearance of (OK I know it was unsupported but I still used it) WM_CONCAT function.
Mike
http://www.strychnine.co.uk<https://clicktime.symantec.com/3XPm8bU2vpgKvqDNUPNxAvd7Vc?u=http%3A%2F%2Fwww.strychnine.co.uk>
Am 18/06/2021 um 18:25 schrieb Frank Gordon <frankagordon_at_gmail.com<mailto:frankagordon_at_gmail.com>>:
In this new scheme, where if the value is already the value that exists don't do the update, how should AFTER UPDATE FOR EACH ROW TRIGGERS work? Ignore the update if the value is being set to "itself"? Would that break some existing applications?
Frank
On Fri, Jun 18, 2021 at 4:02 PM Jonathan Lewis <jlewisoracle_at_gmail.com<mailto:jlewisoracle_at_gmail.com>> wrote:
Mark,
Don't forget the transformation would also have to cater for "column1 is null", because that should be updated too (unless the update were to set it to null).
But the problem is a "notable change in behaviour" - the no-change rows would not be locked. I can't think of an example why this might be bad news in a well-designed and coded sysatem, but imagine:
user A: update rows set ownership = 'USERA' where ...'; user B: update rows set ownership = 'USERB' where ...; user A: update where ownership='USERA'; user B: update where ownership='USERB'
Assuming the ownership updates have some rows in common, and some of those rows are already set to ownership = 'USERA'.
Current implementation:
All userA rows are locked at step 1
User B waits at step 2 for User A to commit, then probably does a "write consistent restart"
Transformed/Block image/Non-locking implementation
User B at step 2 overwrites some rows which should be owned by userA User A at step 3 waits for user B to commit, then probably does a "write consistent restart".
It shouldn't be too hard to produce refine this framework to produce an example where the final values in the rows that both users were interested in change because of the absence of locking.
Regards
Jonathan Lewis
On Fri, 18 Jun 2021 at 15:15, Clay Jackson <dmarc-noreply_at_freelists.org<mailto:dmarc-noreply_at_freelists.org>> wrote:
MWF wrote:
“At the query level I wonder if permuting the query when it can be certainly iso-functional as
update someTable
set someColumn1 = 1.234
becomes
update someTable
set someColumn1 = 1.234
where someColumn1 != 1.234”
I almost can’t believe I’m suggesting this; but is there a nugget of an “enhancement” request in there? I could see cases where if the optimizer was going to do an index lookup (i.e. there was a unique index on that column), OR, a full table scan (i.e. NO other path) a query rewrite might give some dramatic results (I think any paths other than “direct index” or full table scan would probably not be deterministic enough to “take a chance”).
Clay Jackson
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Received on Fri Jun 18 2021 - 20:27:33 CEST